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22nd May 2010, 05:59 PM  #1 
diyAudio Member
Join Date: Mar 2009

Voltage Gain on Long Tailed Drivers
Does anybody here know the typical gain values for a long tail driver/inverter
section as seen in the classic Fenders and Marshall amps? I'm looking for a general range here and the answer can include 12AT7  12AX7 differences as well as small power vs big power. In other words, how much gain(typically) is the driver circuit by itself supplying to the output section. Electronic or in dbs is fine. Thanks 
22nd May 2010, 07:01 PM  #2 
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ

1/4 μ is a good 1st approximation. You do somewhat better, when R_{P} is low.
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Eli D. 
22nd May 2010, 07:11 PM  #3 
diyAudio Member
Join Date: Oct 2004
Location: Maui, Hawai'i, USA

Voltage gain of a singlydriven diff amp is expressed as:
1/2 ((mu*Rl) / (Rl + rp)) where; Rl is the load resistance, typically the load resistor resistance in parallel with the following stage grid resistor, rp is the plate resistance of the device at the quiescent operating point, mu (μ) being the ratio of gm (grid transconductance) to plate conductance(the inverse of plate resistance). Poinz AudioTropic 
22nd May 2010, 08:03 PM  #4 
diyAudio Member
Join Date: Mar 2009

Did some number crunching and everything looks good. Most answers were roughly .25 of mu
That formula was the missing link  I had been trying with the standard single stage formula but that didn't seem right. 28db is higher than I'd expected but it turns out to be just the right number for what I'm trying to do with an insert design. Thanks again 
22nd May 2010, 09:13 PM  #5 
diyAudio Member
Join Date: Oct 2004
Location: Maui, Hawai'i, USA

Av is actually the same as a commoncathode (plate loaded) amp with a fully bypassed (or no) cathode resistor, from plate to plate. Voltage gain of each device is one half that. Whether it's a quarter of μ depends on the load resistance in relation to the plate resistance. I get more like a third of μ usually, because I don't like to load the devices heavily.
Aloha, Poinz AudioTropic 
23rd May 2010, 10:05 PM  #6 
diyAudio Member
Join Date: May 2010
Location: Ball Ground, GA

Using the classic 12AT7 circuit from the early BF period (plate loads = 82K & 100K, 1M grid resistors, 470 ohm cathode resistor, 22K tail resistor), the gain from the 12AT7 input grid to either output tube is 19.25. This can vary 23% based on different tube examples, but otherwise you can take this to be an accurate value, based on measurements taken years ago in a test cell model, where precision parts were used, and the measurements taken.
Dave 
24th May 2010, 02:06 AM  #7  
diyAudio Member

Quote:
Once you have that, the gain per phase of your LTP will come in as half of that singleended gain. With LTPs, half the input voltage appears as Vgk, and half appears across the tail load. In this case, the loadline predicts Av= 50, singleended. When this was actually constructed, the measured gain per phase turned out to be Av= 25 / phase  a 0% error! 

24th May 2010, 04:41 AM  #8 
diyAudio Member
Join Date: Mar 2009

Thanks for posting that chart it happens to be for the exact tube in question in the project I'm working on.

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